Calls are coming in from around the world to immediately end all fighting in a key Sudan border state as thousands flee to escape the violence.

The United Nations said Saturday at least 3,000 people have crossed into Ethiopia since clashes erupted in Blue Nile state between the Sudanese army and rebels loyal to newly independent South Sudan.

U.N. refugee chief Antonio Guterres said the fighting must end immediately to avoid "yet one more refugee crisis" in a region that already suffers from multiple conflicts and neighbors the famine- and conflict-stricken Horn of Africa.

Guterres' call echoes similar statements from U.N. Secretary-General Ban Ki-moon and the U.S. state department Friday. Both said they were "deeply concerned" about the hostilities and called on Sudan and South Sudan to work on resolving differences through peaceful negotiation.

Fighting broke out Thursday in Blue Nile state, which is in Sudan, but home to many who supported the south during a 21-year civil war.

Each side has accused the other of instigating violence in the town of al-Damazine.

Sudan's state run news agency said Friday the government had responded by declaring a state of emergency in Blue Nile state and appointing a military ruler for the region.

The latest unrest has raised concerns that fighting between the two sides is spreading from Sudan's Southern Kordofan state, which is also located along the border. Clashes have escalated since South Sudan declared independence from the north in July.

Sudanese President Omar al-Bashir called for a truce in Southern Kordofan state in late August, after the United Nations said it had reports indicating that Sudanese forces were committing human rights abuses against southern supporters.